R TEPHRA FROM MOUNT RAINIER, WASHINGTON STATE, USA: GEOCHEMICAL CHARACTERIZATION AND DATING OF A POST-GLACIAL MARKER BED

Mount Rainier, located in Washington State, USA, is a stratovolcano that forms part of the Cascade volcanic arc. Mount Rainier has erupted repeatedly since the last glacial period, depositing at least twenty tephra units. The R tephra is the oldest recognized post-glacial lapilli tephra at Mount Rainier, and forms a marker bed east to northeast of the volcano dating to the Pleistocene-Holocene transition in sedimentary records. We collected samples from Tipsoo Lake 19 km east of the summit using a modified Livingstone piston-coring apparatus, and from a stream-cut exposure in the Cowlitz Divide area 12 km southeast of the summit. Field evidence from these locations and several others within Mount Rainier National Park suggests that the tephra includes at least two primary lapilli-bearing layers which appear as coarse yellowish-orange horizons in alpine meadow exposures and gray-brown horizons in subaqueous sediment cores. Observed horizon thicknesses range from a few centimeters in lacustrine deposits up to 20 cm in meadow deposits. Petrographic and geochemical analyses conducted at two independent laboratories provide the first robust suite of geochemical data that characterize and fingerprint the tephra. Electron microprobe analyses of glass shards reveals that glass is heterogeneous with populations clustering at 61 – 68% SiO₂ and 70 – 74% SiO₂. These are linked by intermediate compositions that may reflect magma mixing or fractionation. The mineral assemblage of phenocrysts in lapilli-sized tephra consists of clinopyroxene, orthopyroxene, plagioclase and magnetite, with trace apatite, ilmenite and olivine. Weathering indices suggest that subaqueous R tephra from Tipsoo Lake is slightly more weathered than subaerial R tephra from the Cowlitz Divide area, but the results are within analytical uncertainty. Statistical analysis of two previously published radiocarbon ages and two new radiocarbon ages (this study), providing upper and lower age constraints, show that the deposition of R tephra occurred between 10,000 and 10,100 cal yr BP at 68% probability and between 9950 and 10,140 cal yr BP at 95% probability. The median age for R tephra is ~10,050 cal yr BP.